On Wed, 10 Jul 2019, Paolo Bonzini wrote: > On 08/07/19 21:32, Lendacky, Thomas wrote: > >> AMD and Intel both have serializing lfence (X86_FEATURE_LFENCE_RDTSC). > >> They've both had it for a long time, and AMD has had it enabled in Linux > >> since Spectre v1 was announced. > >> > >> Back then, there was a proposal to remove the serializing mfence feature > >> bit (X86_FEATURE_MFENCE_RDTSC), since both AMD and Intel have > >> serializing lfence. At the time, it was (ahem) speculated that some > >> hypervisors might not yet support its removal, so it remained for the > >> time being. > >> > >> Now a year-and-a-half later, it should be safe to remove. > > > > I vaguely remember a concern from a migration point of view, maybe? Adding > > Paolo to see if he has any concerns. > > It would be a problem to remove the conditional "if > (boot_cpu_has(X86_FEATURE_LFENCE_RDTSC))" from svm_get_msr_feature. But > removing support for X86_FEATURE_MFENCE_RDTSC essentially amounts to > removing support for hypervisors that haven't been updated pre-Spectre. > That's fair enough, I think.
Yes, they have other more interesting problems :)
